Investigating Slips and Falls:
The Complex Dynamics Behind Simple
by Barrett C. Miller
Falls are consistently the leading cause of injury-producing
accidents. They account for more than 1 million injuries each year in the United States.
Why are there so many falls? In part, it is because we fail to
understand the complexity of same-surface slip-and-fall accidents. We assume that people
fall because the floor is slick, because they are clumsy or careless, or because they step
on a foreign object. These assumptions lead to investigations that are one-dimensional and
to repeated accidents at the same location.
To understand causality and to determine responsibility for an
accident, the investigator usually eliminates many possible causes. But people can fall
for numerous reasons. Reasons include the interaction of the walking surface with shoes;
the environment, along with its distractions; and the physical and mental limitations of
The Mechanics of Walking
The study of energy, as applied to motion, is called kinesiology.
Kinesiology provides us with a model to understand normal and abnormal movement patterns.
Any movement, from a golf swing to a child climbing a flight of stairs, can be analyzed.
Most movements involve the expansion and contraction of muscles in
relation to joints and bones. People use levers to move their bodies. When we understand
the patterns in which these levers work, we can determine the cause of deformities and
incapacitates of a person in an accident. We cannot assume, however, that abnormal
patterns of movement are responsible for an accident.
Walking and most other motions of the whole body involve the body's
center of mass. This theoretical area, commonly called "the center of gravity"
(COG), is the balance point around which a movement operates. While complex measurements
are necessary to determine the exact location of the center of gravity, it is estimated
that the COG in the average person who is standing is about 55 percent of the distance
between the floor and the person's height.
This COG is located in the center of the body, as viewed from the
side. The location of the center of gravity affects the way a person walks and falls, and
may even affect the severity of a fall.
The COG changes during various activities and postures, and also
varies according to the build of the person. In walking, the center of gravity is carried
alternately over the right and left foot. The average human walking pattern is called
"striding bipedalism" because we stand and walk with two feet in contact with
the walking surface.
Each step begins with the weight supported for the trailing foot. We
swing the striding foot forward and begin to transfer our weight forward to the heel. Our
weight rolls toward the toes as our momentum carries us forward, and we begin to swing the
alternate leg forward. As the pace increases, an instance of instability occurs between
steps at the moment when our weight is transferred to the leading foot.
Essentially, walking results in successive losses of balance. Clayne
Jensen, a motion expert, divides the human stride into three parts: propulsion, swing, and
catch and support. (C. Jensen, Applied Kinesiology and Biomechanics (1982).) When
speed increases to running pace, only one foot is in contact with the surface at a time.
People walk in distinctive ways. There are so many variations of
walking that some contend there is no normal walking pattern. For example, some people
nearly skip the heel in the forward step, and obese people often step flat-footed because
their center of gravity is carried forward. Each walking pattern causes variations in
These methods of walking are generally thought to be incorrect
because they are less efficient, but they should not be considered unsafe unless evidence
shows they prevent the individual from maintaining balance. Stride and balance can also be
affected by the inner ear, medication, and disorders of the central nervous system.
To stand, walk, or climb without falling, we must maintain our
center of mass over and within a base area. When someone is standing erect, the base can
be considered the normal footprint. The shape, size, and position of the base changes
depending on the pattern of movement and the activity. When walking, we constantly
readjust our body segments over our base to maintain stability. The brain, vision, body
condition, and the nature of the contact with the surface all contribute to the sensitive
balance required to maintain walking stability. If a foot slips or is mispositioned, the
center of gravity shifts outside the base area. When this happens, we shift our body parts
in an attempt to regain equilibrium. If the center of mass cannot be shifted back over the
base area, we fall.
Walking is a motor skill. Once learned, motor acts like walking are
initiated in the cerebral cortex without conscious intention or intervention. Obviously,
we do not need to tell our body what joint to move or which muscles to energize. The mind
orders a whole action, and the details occur without conscious direction.
A national champion golfer was asked to describe the steps involved
in making a shot. She said, "I see the shot, then feel it, and then I do it."
Walking is the same.
Ordinarily, we do not consciously recognize changes in the walking
surface. As we walk, we scan about 10 to 20 feet ahead of us. Our observations operate
below the level of the conscious mind.
Most slip-and-fall accidents are caused by unexpected changes in the
walking surface. To become aware of the hazard, the change must impinge upon our
consciousness in some way.
If we see a potential hazard, we can usually avoid it. It is
possible to walk safely on an icy sidewalk, for example, if we see the hazard and adjust
our behavior. It's the unseen hazard that places us at maximum risk.
When walking in a public place, we have the right to be advised of any
hazards we are likely to encounter. If we choose to proceed with full knowledge of a
hazard, we assume at least some risk. If the hazard has been knowingly or carelessly
disguised, an accident can be expected.
A man visiting a public park broke his leg in a fall as he tried to
descend three wooden steps to a lower level. His foot had slipped on a defect in the
leading edge of a step. The public parks department investigated the accident and painted
the steps with a bright orange paint to call attention to the stairs. unfortunately, the
paint covered the defect in the step, and, two weeks later, a woman broke her hip when her
foot got caught in the same defect.
Lighting can disguise a defect or hazardous condition. A change in
the normal walking environment must be visible to the pedestrian and stand out from
background stimuli. glare and too much or too little contrast in the walking environment
can reduce the efficiency of the eye.
The walking surface should be evenly illuminated and should have a
brightness level of at least 20 foot candles - a basic quantitative unit of light
measurement. And the contrast (ratio of dark to light) should be no less than 3-1 and no
more than 20-1. The measurement of luminosity and contrast requires a simple photographic
light meter calibrated to read in foot candles.
Photographs of the accident scene can help the investigator evaluate
the effect of light on the mishap. It is important to photograph the site properly, making
sure the photographs represent the accident point from the eight compass positions. These
photographs often provide unexpected insights. The camera should be held level and pointed
straight ahead, instead of at the floor.
An example of the usefulness of photographs can be shown in the case
of a woman who fell while walking through a bank. An examination of the photographs showed
what had been missed during previous inspections of the scene: a dip in the floor, where a
wall had been removed, which could not be seen from the area of traffic flow used by the
Kinds of Falls
Same-surface falls can be classified into four categories:
1. Trip-and-fall accidents, in which pedestrians encounter a foreign
object in their walking path;
2. Stump-and-fall accidents, in which a moving foot encounters an
impediment in the walking surface, whether it is a tacky point on the surface or a defect
that impedes the foot;
3. Step-and-fall accidents, in which the foot finds an unexpected
failure or hole in the walking surface; and
4. Slip-and-fall accidents, in which the interface of the shoe and the
floor fails to support the walker's center of gravity over the base area.
The slip and fall is the most common accident. Foot contact is
broken, and the individual attempts to right himself or herself. Recovery of equilibrium
is reflexive and not under conscious control in most cases. If the pedestrian strikes the
surface with a fleshy part of the body, the injuries are likely to be minimal. but if the
victim strikes a bony body part, the injuries may be more severe.
Measurement of Surfaces
Years before the invention of modern acrylic finishes and attempts to
establish standards for regulation of surface slipperiness, the cleaning industry
discovered the need to determine slip-resistance.
The bean-bag test was one of the earliest ways, and it can be traced
back to the 1930s. A 10-pound bag of beans was placed on a clean piece of burlap. The
weighted burlap was pulled across the floor with a spring scale. The point at which the
load began to move was noted, and the test was repeated.
A floor that required six pounds of pull was considered safe. A
floor with less than five was slippery, and a floor with a drag of more than seven was
considered to be tacky. By today's standards, the bean-bag test is primitive, but it
provided a simple, reliable way of comparing results and surfaces. A new product could be
tested on a small area, and the safe working procedures could be recorded.
The American Society for Testing and Materials and the National
Bureau of Standards have studied the reliability of test devices for measuring the floor
surfaces and the effect of cleaning materials on walking safety. We can now predict how a
floor-care product will behave when applied according to a manufacturer's recommendations.
The new tests are merely revisions of the old bean-bag test.
Floor characteristics are now measured in terms of the static or
dynamic coefficient-of-friction(COF). Most common machines measure the
coefficient-of-friction of a surface by dragging, pushing, or striking a floor area with a
shoe material having a known surface characteristic. This coefficient can be determined by
taking the drag necessary to first move the object and dividing it by the weight of the
object being pulled. a floor with a COF of .5 or less is considered slippery; a floor with
a COF of more than .5 is considered safe.
For forensic analysis of floor materials, Seigler Pendulum Tester, a
horizontal drag slip meter, or another type of calibrated test device is often used. These
systems of testing floor surfaces depend on a heel and sole material of known slip
characteristics, and the accuracy of the systems has been validated by the National Bureau
of Standards and other testing agencies. These simple devices can be very useful. They
provide the safety investigator a standard with which to compare a floor surface.
Modern materials and floor-care products are now considered safe and
consistent, so builders and service companies no longer test each batch of a product. But
this confidence can lead to problems.
A new shopping center in a tropical city, for example, used glazed
ceramic tile in its walkway. A woman slipped on the tile and was hurt. The architect had
specified an eight-inch strip of ceramic tile with a decorative pattern, and the builder
presented the owner with samples. No one knows if the tile manufacturer had recommended
the use of this tile on outside walkways, but it was a poor choice. When it rains, as it
often does in this tropical city, the sidewalk becomes hazardous.
The static coefficient-of-friction has been measured at .30 on a wet
day. In this case, a small number of tiles could have been tested under conditions similar
to those in its expected use. The cost wold have been minimal. but now the tile must be
completely removed from the shopping center or temporarily fixed by applying an external
finish. The removal will cost thousands of dollars.
Types of Floors
The slip-resistance of floor materials has received considerable
attention from safety investigators. There is a large body of knowledge about material
characteristics. Each material has benefits and disadvantages, and each must be properly
installed and maintained to produce a safe walking surface.
As a rule, the slip-resistance of a floor material is directly
proportional to the number of microscopic points that project from its surface. Concrete,
for example, is a relatively safe surface, but if it is improperly cured when poured, it
becomes a very slippery surface.
Asphalt tile and vinyl tiles are relatively safe if the surface
treatment chosen is correct. Marble and terrazzo surfaces are inherently slippery and
should be treated with a sealer containing a high percentage of solids to increase
slip-resistance. Floor tile can be very safe or very dangerous, depending on its factory
finish and its maintenance.
Floor Finishes and Cleaning Methods
A custodian's locker is a good place to start an investigation of
slip-and-fall accidents. The investigator should determine what products are being used
and how they are being combined. Are the mops dirty? Does the staff use the same mop for
cleaning and disinfecting? How often is the floor finish removed, and what is used to
A floor finish is a product that is put on top of the raw floor
material to protect it, to beautify it, or to change its surface characteristics. A floor
finish can be used to make an unsafe floor less slippery or can inadvertently be used to
make a safe floor dangerous. Finishes can be either synthetic or organic.
One potentially dangerous floor finish is a paste produce called
"carnuba." Carnuba, made from the palm tree, gives the floor a brilliant shine
and is preferred by some floor-care companies because of its ease of use, shine, and cost.
When used according to manufacturer's recommendations, the product is safe and offers and
acceptable standard of floor care.
If problems arise, it is usually because the produce is used
improperly. The wax material in carnuba is soft and "walks," that is, it
gravitates out of the pedestrian path. The custodian often replaces the thin areas with
fresh paste, leaving a heavy build-up toward the outside of the traffic area. This
build-up produces a dangerous surface.
Acrylic finishes are also common. Some companies offer special
formulations for problem floor surfaces like terrazzo and marble. One manufacturer makes
an acrylic finish that contains aluminum oxide flakes. This finish can be applied in
restaurants and other locations where floors are usually wet. These acrylics form a
one-piece floor surface through interlocking crystallization when the material dries.
Acrylics make safe floor surfaces, but are labor intensive and have to be removed when the
product builds up.
Improper cleaning methods can complicate matters further. Oil mops
treated with petroleum base sprays can turn a safe floor into a hazardous one. Pine-tar
disinfectants used on ceramic floors sometimes leave a slippery residue and cause many
The National Safety Council reports that may slip accidents are
caused by improper cleaning methods and recommends that floors be cleaned only with clean
water. If soap or commercial strippers are used on a floor, care must be taken that no
residue remains when the floor dries.
A final finish is sometimes applied to the floor by the pedestrian,
who can pick up fertilizer and other chemicals on shoes and bring dangerous adherents into
Individuals at Risk
If a victim has been previously injured, is disabled in some other way,
or is elderly, we might assume that the physical infirmity caused the accident. But our
assumptions can prevent our seeing the actual cause. There is no correct way to stand or
walk. What we view as correct posture and movement is often a matter of esthetics.
Handicapped people sometimes need to use a high level of conscious intervention in the
Mrs. Jones is an example. She suffered from polio as a child, and
her leg muscles and hip were seriously atrophied. She used leg braces and a cane when she
walked. For 35 years, she had walked on ice and snow and climbed stairs and moved about in
all kinds of situations. She walked monopedially, that is, she often had only one foot in
contact with the floor when she took a step. At the moment when she shifted her weight
from one foot to the other, she used the cane for stability.
Mrs. Jones entered a fast food restaurant that displayed a sign
indicating it was equipped for handicapped use. It was a bright day, and the restaurant
was using subdued lighting for effect. The windows had been treated to cut down light
transmission. When entering this environment, it takes between three to five seconds for
one's eyes to adjust to the light. For Mrs. Jones, the light at floor level was less than
20 foot candle, and the contrast between the brown ceramic tile and a spilled soft drink
was too low for her eyes to measure. Her right foot slipped to the right, and she fell. A
close examination showed that the pores of the normally safe unglazed ceramic tile were
packed with soap residue. When wet, the spot produced a dangerous condition, and the
subdued lighting hid the condition.
Investigators of falls should understand the principles of human
movement and have a working knowledge of floor materials, cleaning methods, and lighting.
When reconstructing a slip-and-fall accident, investigators should
avoid coming to any conclusion too quickly. The victim should be extensively interviewed,
and the verbal report used to focus the investigation. The slip-resistance of the floor
should be measured, and accurate measurements of the light intensity and contrast should
In every case, the findings should be compared to building codes and
industry standards. Although they are seemingly the simplest of all accidents, falls are